Digestion, Absorption, Transport,

and

Excretion of Nutrients

(Session 6)

Mohsen Karamati Department of Nutrition Sciences, Varastegan Institute for Medical Sciences, Mashhad, Iran

E-mail: karamatim@varastegan.ac.ir

Digestion and Absorption of

Specific Types of Nutrients

Carbohydrates and Fiber

Most dietary carbohydrates are consumed in the form of

starches, disaccharides, and monosaccharides.

Starches, or polysaccharides, usually make up the greatest

proportion of carbohydrates.

Starches are large molecules composed of straight or

branched chains of sugar molecules that are joined together,

primarily in α-1-4 or α-1-6 linkages.

Most of the dietary starches are amylopectins, the branching

polysaccharides, and amylose, the straight chain-type

polymers.

Structure of starch: (A) Amylose, showing helical coil structure; (B)

Amylopectin, showing 1→6 branch point

Dietary fiber is also largely made of chains and branches of

sugar molecules, but in this case the hydrogens are

positioned on the beta (opposite) side of the oxygen in the

link instead of the alpha side, resulting in its resistance to

enzymatic digestion in human GIT.

That humans have significant ability to digest starch, but not

most fiber, exemplifies the "stereospecificity" of enzymes.

Digestible

Indigestible

Indigestible

In the mouth, salivary amylase (ptyalin) operates at a

neutral/slightly alkaline pH and hydrolyzes a small amount

of the starch molecules into smaller fragments, and then

becomes deactivated after contact with HCl.

The stomach empties before significant carbohydrate

digestion can take place; thus, most carbohydrate digestion

occurs in the proximal small intestine.

Pancreatic amylase breaks the large starch molecules at α-1-

4 linkages to create maltose, maltotriose, and “alpha-limit”

dextrins remaining from the amylopectin branches.

Enzymes from the brush border of the enterocytes further

break the disaccharides and oligosaccharides into

monosaccharides glucose, galactose, and fructose.

The brush border of the enterocytes contain the enzymes

maltase, sucrase, lactase, and isomaltase (or a-dextrinase),

which act on maltose, sucrose, lactose, and isomaltose,

respectively.

Glucose, galactose, and fructose pass through the mucosal

cells and into the bloodstream via the capillaries of the villi,

where they are carried by the portal vein to the liver.

At low concentrations, glucose and galactose are absorbed

by active transport, primarily by a sodium -dependent

transporter called the glucose (galactose) cotransporter.

At higher luminal concentrations of glucose, glucose

transporter (GLUT) 2 becomes a primary facilitative

transporter into the intestinal cell.

Fructose is more slowly absorbed and uses GLUT5 and the

facilitative transporter from the lumen.

GLUT2 is used to transport glucose, galactose, and fructose

across the intestinal cell membranes into the blood.

The sodium-dependent transport of monosaccharides is the

reason why sodium-glucose drinks are used to rehydrate infants

with diarrhea or athletes who have lost too much fluid.

Consumption of large amounts of lactose (especially in

individuals with a lactase deficiency), fructose, stachyose,

raffinose, or alcohol sugars (e.g. sorbitol, mannitol, or xylitol)

can result in considerable amounts of these sugars passing

unabsorbed into the colon and may cause increased gas and

loose stools.

Cellulose, hemicellulose, pectin, gum, and other forms of fiber

cannot be digested by humans; thus these carbohydrates pass

relatively unchanged into the colon, where they are partially

fermented by bacteria in the colon.

Other resistant starches and sugars are also less well digested or

absorbed by humans, but are fermented into SCFAs and gases.

It is noteworthy that one form of dietary fiber, lignin, is made of

cyclopentane units and is neither readily soluble nor

fermentable.

Proteins

Protein digestion begins in the stomach, where some of the

proteins are split into polypeptides by the enzyme pepsin.

Inactive pepsinogen is converted into the enzyme pepsin

when it contacts HCl and other pepsin molecules.

Unlike any of the other proteolytic enzymes, pepsin digests

collagen, the major protein of connective tissue.

Contact between chyme and the intestinal mucosa stimulates

the release of enterokinase, an enzyme that transforms

inactive pancreatic trypsinogen into active trypsin (i.e. the

major pancreatic protein-digesting enzyme).

Trypsin in turn activates the other pancreatic proteolytic

enzymes.

Pancreatic trypsin, chymotrypsin, and carboxypeptidase

break down intact protein to small polypeptides and amino

acids.

Proteolytic peptidases located on the brush border also act

on polypeptides, breaking them down into amino acids,

dipeptides, and tripeptides.

The final phase of protein digestion takes place in the brush

border, where some of the dipeptides and tripeptides are

hydrolyzed into amino acids by peptide hydrolases.

End products of protein digestion are absorbed as both

amino acids and small peptides.

Several transport molecules (sodium- or chloride-dependent,

etc.) are required for the absorption of amino acids, probably

because of the wide differences in the size, polarity, and

configuration of the different amino acids.

Considerable amounts of dipeptides and tripeptides are also

absorbed into intestinal cells using a peptide transporter, a form

of active transport.

Almost all protein is absorbed by the time it reaches the distal

jejunum, thus only 1% of ingested protein is found in the feces.

Absorbed peptides and amino acids are transported to the liver

via the portal vein.

Small amounts of amino acids may remain in the epithelial cells

for synthesis of new proteins (e.g. intestinal enzymes).

Lipids

Approximately, 97% of dietary lipids are in the form of

triglycerides, and the rest are phospholipids and cholesterol.

Only small amounts of fat are digested in the mouth with

lingual lipase and in the stomach with gastric lipase or

tributyrinase, which hydrolyzes short-chain triglycerides (such

as those found in butter), into fatty acids and glycerol.

Most fat digestion takes place in the small intestine as a

result of the emulsifying action of bile salts and hydrolysis

by pancreatic lipase.

Entrance of fat and protein into the duodenum stimulates

the release of CCK (which stimulates biliary and pancreatic

secretions) and enterogastrone, which inhibit gastric

secretions and motility, thus slowing the delivery of lipids.

Therefore, a portion of a large, fatty meal may remain in the

stomach for 4 hours or longer.

The peristaltic action of the small intestine and the

emulsification action of bile reduces the fat globules into tiny

droplets, thus making them more accessible to digestion by

the pancreatic lipase.

The free fatty acids and monoglycerides produced by

digestion form complexes with bile salts called micelles,

which facilitate passage of the lipids through the watery

environment of the intestinal lumen to the brush border.

Bile is a liver secretion composed of bile acids (primarily

conjugates of cholic and chenodeoxycholic acids with glycine or

taurine), bile pigments (which color the feces), inorganic salts,

some protein, cholesterol, lecithin, and many compounds such

as detoxified drugs.

From its storage organ, the gallbladder, approximately 1 L of

bile is secreted daily.

Most of the bile salts are actively reabsorbed in the terminal

ileum and returned to the liver to reenter the gut in bile

secretions through the process of enterohepatic circulation.

In the mucosal cells the fatty acids and monoglycerides are

reassembled into new triglycerides.

These triglycerides, along with cholesterol, fat-soluble

vitamins, and phospholipids, are surrounded by a

lipoprotein coat, forming chylomicrons.

Chylomicrons pass into the lymphatic system and are

transported to the thoracic duct and emptied into the

systemic circulation at the junction of the left internal

jugular and left subclavian veins.

The chylomicrons are then carried through the bloodstream

to several tissues, including liver, adipose tissue, and muscle.

In the liver, triglycerides from the chylomicrons are

repackaged into very low-density lipoproteins and

transported primarily to the adipose tissue for metabolism

and storage.

The fat-soluble vitamins A, D, E, and K are also absorbed in a

micellar fashion, although water-soluble forms of vitamins A, E,

and K supplements and carotene can be absorbed in the absence

of bile acids.

Normally, 95% to 97% of ingested fat is absorbed into lymph

vessels.

Increased motility, intestinal mucosal changes, pancreatic

insufficiency, or the absence of bile can decrease fat absorption.

When undigested fat appears in the feces, the condition is

known as steatorrhea.

Because of their shorter length and thus increased solubility,

fatty acids of 8 to 12 carbons (i.e. medium-chain fatty acids)

can be absorbed directly into mucosal cells without the

presence of bile and micelle formation, where they are able

to go directly without esterification into the portal vein.

Due to these characteristics, medium-chain triglycerides

(MCTs), which have medium-chain fatty acids, are clinically

valuable for individuals who lack necessary bile salts for long-

chain fatty acid metabolism and transport.

Summary of fat digestion and absorption

Vitamins and Minerals

Vitamins and minerals from foods are made available as

macronutrients and are digested and absorbed primarily in

the small intestine.

At least some vitamins pass unchanged from the small

intestine into the blood by passive diffusion, but several

different mechanisms might be used to transport individual

vitamins across the GI mucosa.

Mineral absorption is more complex, especially the

absorption of the cation minerals.

These cations, are made available for absorption by the

process of chelation, in which a mineral is bound to a ligand,

usually an acid, an organic acid, or an amino acid, so that it

is in a form absorbable by intestinal cells.

References:

1- Beyer PL. Intake: Digestion, Absorbtion, Transport, and

Excretion of Nutrients. In: Mahan LK, Escott-Stump S,

Raymond JL, editors. Krause's food & the nutrition care

process. 13th ed. USA: Elsevier; 2012: 2-18.

2- Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell

VW, Weil PA, editors. Harper's Illustrated Biochemistry. 28th

ed. USA: McGraw-Hill; 2009.

3- Ross AC, Caballero B, Cousins RJ, Tucker KL, Ziegler TR,

editors. Modern Nutrition in Health and Disease. 11th ed. USA:

Lippincott Williams & Wilkins; 2014.

4- Wikipedia, the free encyclopedia. Available from: URL:

http://en.wikipedia.org